The present invention relates to an optical scanning device and, more particularly, to a scanning device in which only the area of the document being scanned is illuminated.
The optical scanning of a document is a necessary part of the operation of the number of types of devices, including copiers, duplicators, and facsimile transmission devices. Where the image on the document is to be converted into electrical signals, it is common to use photosensitive transducers, such as photodiodes. A transducer of this sort provides an electrical resistance which varies as a function of the amount of light striking the transducer. Typically, the document to be scanned is illuminated and light reflected from the document is directed to one or more such photodiodes by means of scanning optics. The scanning optics are continuously adjusted so that light relected from successive areas across the document will be directed onto the photodiodes.
As the photodiodes receive a number of photons, their resistance changes accordingly. A certain minimum of number of photons must strike the photodiodes, however, for the resistance of the diodes to be altered. With a given intensity of illumination for the image on the document being scanned, therefore, it will be appreciated that the maximum scanning rate of the document is limited by the photosensitivity of the transducer. In order to provide sufficient image resolution at high scan rates, it is necessary to increase the illumination of the master document.
Illuminating the entire document with a large quantity of light, however, has certain drawbacks. If too much light is directed at the document, the document may be adversely affected, as by singeing or discoloring, depending upon the document material. Additionally, it will be appreciated that substantial increases in illumination result in corresponding increases in power consumption. Finally, use of a large number of light sources, such as quartz lamps, for providing high intensity lighting over the entire surface of the document presents substantial cooling problems.
A number of prior art devices have incorporated scanners in which less than all of the original document is illuminated during the scanning operation. U.S. Pat. No. 3,694,070, issued Sept. 26, 1972, to Libby, discloses a copying system which includes scanning illumination elements that focus light on only the area of the original document actually being optically scanned by the system. Rotating reflectors co-operate with stationary lamp elements to project a line of light onto an original document. A rotating mirror scanning arrangement directs light reflected from the original document throught a lens element to an image plane in the copying system for xerographic copying. Scanning of the document and movement of the line of light across the document are accomplished in synchronization. The Libby patent does not disclose how the desired rotation of the reflectors is controlled.
U.S. Pat. No. 3,549,894, issued Dec. 22, 1970, to Ortmann, discloses an optical scanning arrangement in which fixed lamps illuminate a moving scan point. Lenses positioned on the periphery of a rotating scanning wheel focus light onto a document at the scan point. Scanning lenses on the wheel direct relected light to light transducers. A line across the document is therefore scanned by rotation of the lenses past the document. Movement of the document in a direction parallel to the axis of rotation of the scanning wheel results in raster scanning of the entire document.
U.S. Pat. No. 4,041,454, issued Aug. 9, 1977, to Shepard et al discloses a scanning arrangement in which a beam of light scans across a document in raster fashion and is reflected from the document to a single photo-optical transducer. The Shepard et al scanning system requires that a narrow beam of light no larger than the elements into which the image is to be resolved be directed at the master, since the transducer and an associated collecting reflector include no optical focusing elements. The transducer therefore receives all of the light reflected from the document.
U.S. Pat. No. 3,422,216, issued Jan. 14, 1969, to Kyte, discloses a "flying spot" scanner using a scanning beam generated by a cathode ray tube. The spot scans in raster fashion across a transparent film image. Only the light source is moved across the original document. The light receiving optics are fixed in position with respect to the original document and, therefore, the resolution of the image produced is determined by the size of the spot used in scanning.
Thus, it is seen that there is a need for an improved optical scanner in which only an area of the original document corresponding to the area being scanned is illuminated.